U.S. Pat. No. 3,682,609 to Stuart M. Dockerty (the Dockerty patent) describes a system for controlling the thickness distribution across the width of a glass ribbon by locally controlling its temperature. To do so, the Dockerty patent uses a pair of refractory plates or walls whose long axes run parallel to the width of the ribbon. One plate is located on each side of the ribbon and the pair are positioned along the length of the ribbon above the point where the thickness of the ribbon becomes fixed. The plates are placed relatively close to the ribbon so that they can absorb heat from the molten glass.
An array of tubes is located behind each plate and oriented so that fluid (e.g., air) ejected from the tubes impinges on the back of the plate. The fluid flow from each tube is individually controllable. By adjusting the fluid flow from the tubes, the local temperature on the front face of the plate can be controlled. This local temperature affects the local heat loss, and thus the local temperature, of the molten glass, which, in turn, affects the final thickness distribution across the width of the ribbon. In practice, the Dockerty system has proven highly effective in controlling thickness variations across the width of glass ribbons and is widely used in the production of glass sheets for such demanding applications as substrates for liquid crystal and organic light emitting diode displays (LCDs and OLEDs).
As currently practiced, the air flow rates in the tubes of the Dockerty system are adjusted manually by operators. Operators look at a measured sheet thickness trace and use their experience and judgment, or “feel,” to decide which tubes to adjust, and by how much, to eliminate non-uniformities in the thickness trace. This reliance on “feel” causes a variety of problems.
For example, when there is a significant change to the process, such as a higher glass flow rate or a different glass composition, substantial time is often needed during start-up until operators acquire a “feel” for the way the changed process behaves. Furthermore, as thickness variation specifications are tightened, there is no way of knowing whether operator “feel” will be able to meet the new specifications and, if so, how long it will take to do so. Although operator “feel” has worked in the past, it is unclear if it will be up to the challenges imposed by ever more exacting standards for glass sheets, especially those used as substrates for display applications.
More generally, relying on operator “feel” means that new operators must undergo a learning process before they can make sound judgments regarding air flow distributions across the width of the ribbon. With the expanding demand for flat screen televisions and monitors, there may come a time when trained operators becomes a scarce resource limiting the number of glass making machines that can be in operation at any one time.
The present disclosure addresses these problems and provides methods for controlling the temperature distribution across the width of a glass ribbon so that sheet thickness variations are within specifications without the need for trained operators who have a “feel” for the system. Rather, it has been found that an iterative process which does not rely on “feel” can be employed to meet thickness specifications using a small number of iterations provided that each iteration is based on a mathematical analysis (described below) of the thickness behavior produced by the prior iteration.